The disclosures of Japanese Patent Applications No. 2001-234154 filed on Aug. 1, 2001 and No. 2001-233923 filed on Aug. 1, 2001, each including the specification, drawings and abstract, are incorporated herein by reference in its entirety.
1. Field of the Invention
The invention generally relates to a shift control apparatus of an automatic transmission of a motor vehicle, and more particularly to such a shift control apparatus that effects coast downshift of the transmission while the vehicle is kept in a minimal driving state in which a weak engine brake is applied.
2. Description of Related Art
One type of a downshift control apparatus of an automatic transmission of a motor vehicle has been proposed which keeps the vehicle in a weak engine brake state when the automatic transmission is shifted down while the vehicle is coasting or decelerating with the accelerator pedal being released or not operated. An example of such a downshift control apparatus is disclosed in Japanese laid-open Patent Publication No. 11-287317. With this apparatus, certain engine brake force is kept constantly applied to the vehicle during clutch-to-clutch downshifts, whereby the downshifts to low-speed gear ratios do not cause excessive engine brakes or shift shocks.
When the automatic transmission goes through a clutch-to-clutch downshift, a release-side friction element (or offgoing friction element) is released and an engagement-side friction element (or oncoming friction element) is engaged around at the same time, and therefore the transmission tends to suffer from racing (a rapidly increase in the speed) of its input shaft or a temporary drop (or tie-up) of its output torque. In view of these situations, expensive control equipment that operates with considerably high accuracy may be required to control the engaging pressure for the release-side friction element when it is released and the engaging pressure for the engagement-side friction element when it is engaged. With generally available control apparatus for use in standard vehicles, however, the engaging pressures for the friction elements cannot be controlled with sufficiently high robustness with respect to disturbances, such as braking of the vehicle, and shift shocks may not be sufficiently reduced or suppressed.
It is therefore one object of the invention to provide a shift control apparatus of an automatic transmission with which a clutch-to-clutch downshift during coasting of the vehicle is controlled with sufficiently high robustness with respect to disturbances, such as braking of the vehicle. It is another object of the invention to provide a shift control apparatus of an automatic transmission with which a clutch-to-clutch downshift during coasting is controlled without using high-accuracy control equipment, so that shift shocks are sufficiently reduced even in the event of disturbances, such as braking of the vehicle.
To accomplish the above and/or other object(s), there is provided according to a first aspect of the invention a shift control apparatus of an automatic transmission of a motor vehicle to which a torque is transmitted from an engine via a fluid coupling device, the automatic transmission including a plurality of hydraulically operated friction elements, wherein a clutch-to-clutch downshift is carried out during coasting of the vehicle by releasing one of the friction elements as a release-side friction element and engaging another of the friction elements as an engagement-side friction element. A controller of the shift control apparatus detects a difference between input and output rotation speeds of the fluid coupling device, and increases an engine speed by a controlled amount based on the difference between the input and output rotation speeds when the clutch-to-clutch downshift is carried out during coasting of the vehicle, so that the vehicle is brought into a minimal driving state in which the engine speed is slightly higher than the output rotation speed of the fluid coupling device.
With the shift control apparatus constructed as described above, the engine speed is increased by a controlled amount based on the difference between the input and output rotation speeds of the fluid coupling device when a clutch-to-clutch downshift is effected during coasting of the vehicle. Since the clutch-to-clutch downshift is carried out during coasting while the vehicle is maintained in the minimal driving state as described above, the hydraulic pressures applied to the friction elements associated with the downshift can be controlled with high accuracy owing to reduced torque variation, and sufficient robustness with respect to disturbances, such as braking of the vehicle, can be achieved without requiring high-accuracy control apparatus.
According to a second aspect of the invention, there is provided a shift control apparatus of an automatic transmission of a motor vehicle in which a coast downshift is carried out during coasting of the vehicle, the automatic transmission including a plurality of hydraulically operated friction elements. A controller of the shift control apparatus causes the coast downshift to be carried out while the vehicle is kept in a minimal driving state in which an engine speed is slightly higher than an input shaft speed of the automatic transmission, and sets a hydraulic pressure applied to at least one of the friction elements associated with the coast downshift during a shifting period, according to the minimal driving state.
With the shift control apparatus as described above, the hydraulic pressures applied to the hydraulically operated friction elements associated with the coast downshift during the shifting period are set in accordance with the minimal driving state. Thus, the engaging pressures for the friction elements can be appropriately controlled during the shifting period, so that the engaging and releasing actions can be accomplished with high accuracy, irrespective of disturbances such as braking, resulting in sufficiently reduced shift shocks.
According to a third aspect of the invention, there is provided a shift control apparatus of an automatic transmission of a motor vehicle in which a coast downshift is carried out during coasting while the vehicle is kept in a minimal driving state in which an engine speed is slightly higher than an input shaft speed of the automatic transmission, the automatic transmission including a plurality of hydraulically operated friction elements. A controller of the shift control apparatus controls a hydraulic pressure applied to at least one of the friction elements associated with the coast downshift during a shifting period, according to the minimal driving state, and corrects, by learning, the hydraulic pressure for the at least one friction element during the shifting period.
With the shift control apparatus constructed as described above, the hydraulic pressures applied to the hydraulically operated friction elements associated with the coast downshift during the shifting period are set in accordance with the minimal driving state. Thus, the engaging pressures for the friction elements can be appropriately controlled during the shifting period, so that the engaging and releasing actions can be accomplished with high accuracy, irrespective of disturbances such as braking, resulting in sufficiently reduced shift shocks. In addition, correcting the hydraulic pressures by learning reduces or eliminates differences among individual apparatus or chronological changes, thus assuring improved accuracy in the engaging/releasing actions and significantly reduced shift shocks.